Yes. It can be missed especially if the radiologist was not looking for it, it is small enough to fall between the slices as MRI's have a 3-5mm gap, or the scan was not done with contrast, or done with the dynamic technique.
Prolactin can be raised by thyroid, medications and many many things as well as the pit tumor... So you have to find a decent doc to find the real reason.
I hope you are being sent to an endo, and if they find a pit tumor, please find a pituitary center. Get copies of everything.